This invention relates to aluminum alloy sheet products and methods for making them. Specifically, it relates to a process for manufacturing a new aluminum alloy foil re-roll strip and foil stock using a continuous strip casting process.
Thin gauge or converter foils are generally prepared by casting an ingot of an aluminum alloy such as AA1145 in a process known as DC or direct chill casting. The ingots are generally heated to a high temperature, hot rolled to a re- roll gauge thickness of between 1 and 5 mm, then cold rolled to a "foil-stock" gauge typically 0.2 to 0.4 mm thick. The strip is often subjected to an interanneal step during the cold rolling process. The "foil-stock" is then subject to further cold rolling operations, often using double rolling techniques to produce a final foil thickness of about 5 to 150 microns. When rolled to a thickness of between 5 and 10 microns, the final foil product is frequently referred to as converter foil and is used in various packaging applications.
There is a cost advantage to using continuous strip casting as the starting point in manufacture of such foils since homogenization prior to hot rolling is not required, and the amount of hot reduction to form re-roll gauges is greatly reduced. However, continuous strip casting processes apply different cooling conditions during solidification from those in DC casting, and there is an absence of a high temperature homogenization step prior to hot rolling. Consequently when continuous strip casting processes are used with alloys normally prepared by DC casting and homogenization, this results in the formation of different intermetallic species and shell distortion in the cast product which cause surface defects in the final foil stock product. In continuous strip casting, the cooling rate of the strip during casting is generally higher (in some cases much higher) than the cooling rate in large DC ingots. Thus, such alloys processed in a continuous strip casting process also result in foil stock which has a higher supersaturation of solute elements, and therefore has undesirable hardening and softening properties, resulting in difficulties in rolling the foil stock to the final gauge thickness.
A previous method of manufacturing aluminum alloy strip suitable for use in the production of thin gauge foils is described in Furukawa, Laid-Open Japanese Application 6-63397, published Apr. 5, 1994. This document indicates that a wide range of iron and silicon concentrations may be present in the aluminum alloy, e.g. 0.2-0.8% Fe and 0.05-0.3% Si. However, the highest concentrations of silicon tested were not above 0.19%.
It is an object of the present invention to provide a continuous strip casting based process for preparing re-roll and foil stock suitable for trouble-free production of thin foils with no surface defects such as blemishes and streaks.
It is a further object to produce a foil stock in continuous strip form which contains a substantially single intermetallic species of alpha-phase. "Alpha-phase" means an intermetallic phase consisting of Al--Fe--Si where Fe lies in the range of 30 to 33% and Si lies in the range of 6 to 12% (balance Al). The stoichiometry is typically Fe.sub.3 Si.sub.2 Al.sub.12 to Fe.sub.2 SiAl.sub.8.